Field of the Invention
The invention relates generally to earth-boring drill bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the invention relates to an improved cutting structure for such bits. Still more particularly, the present invention relates to polycrystalline diamond compact cutter elements with improved toughness and thermal stability.
Background of the Invention
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.
Many different types of drill bits and cutting structures for bits have been developed. Two predominant types of drill bits are roller cone bits and fixed cutter bits, also known as rotary drag bits. A common fixed cutter bit has a plurality of blades angularly spaced about the bit face. The blades generally project radially outward along the bit body and form flow channels there between. Cutter elements are typically mounted on the blades.
The cutter elements disposed on a fixed cutter bit are typically formed of extremely hard materials and include a layer of polycrystalline diamond (“PD”) material. In the typical fixed cutter bit, each cutter element comprises an elongate and generally cylindrical support member which is received and secured in a pocket formed in the surface of one of the several blades. In addition, each cutter element typically has a hard cutting layer of polycrystalline diamond or other super-abrasive material such as cubic boron nitride, thermally stable diamond, chemically modified or doped diamond, polycrystalline cubic boron nitride, or ultra-hard tungsten carbide (meaning a tungsten carbide material having a wear-resistance that is greater than the wear-resistance of the material forming the substrate) as well as mixtures or combinations of these materials. The cutting layer is exposed on one end of its support member, which is typically formed of tungsten carbide. For convenience, as used herein, reference to “PDC bit” or “PDC cutter element” refers to a fixed cutter bit or cutting element employing a hard cutting layer that contains polycrystalline diamond (PDC refers to Polycrystalline Diamond Compact).
The cost of drilling a borehole for recovery of hydrocarbons is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness. The length of time that a drill bit may be employed before it must be changed depends upon a variety of factors. These factors include the bit's rate of penetration (“ROP”), as well as its durability or ability to maintain a high or acceptable ROP. In turn, ROP and durability are dependent upon the cutter elements' abrasion resistance, toughness and ability to resist thermal degradation.
Manufacturing polycrystalline diamond requires high pressure and high temperature. Initially, pressure is increased causing the diamond crystals to be pushed against each other with increasing force. These particles move relative to each other and often fragment, increasing the powder apparent density. A coarse powder displays a higher degree of crushing than a finer one, as the average number of contact points per unit volume is much higher for fine powders, and therefore fine powders display a lower contact stress and lower probability for fragmentation.
Secondly, during manufacturing, when the compacted powder is under full pressure, the temperature is raised. The diamond powder is typically packed against a WC—Co substrate, the origin of the catalyst metal (Co) that induces sintering. When the cobalt reaches its melting point, it is forced into the open porosities left within the layer of compacted powder. Sintering takes place through carbon dissolution and precipitation and reduction of internal energy. Densification is determined by the pressure and by the contact area relative to the cross-sectional area of the particles. The reaction speed is proportional to the temperature and to the average effective pressure, which is the actual contact pressure between particles. The sintering process is therefore faster if both the contact pressure and the temperature are increased. Smaller grain size and better packing result in lower contact pressure; therefore sintering PDC of very small particle size requires higher pressures and temperatures.
The smaller the size of the diamond crystals sintered together, the higher the wear abrasion resistance, but the lower the impact strength of the resulting PDC. With larger diamond particle sizes a lower abrasion resistance is observed, but an increased toughness is achieved. Diamond compacts have limited heat resistance and experience high thermal wear. At atmospheric pressure, a diamond's surface turns to graphite at 900° C. or higher. In a vacuum or in inert gas, diamond does not graphitize easily, even at 1,400° C. However during use, conventional PDC cutters experience a decline in cutting performance around 750° C., which the cutting edge can easily reach due to frictional heating in hard, abrasive rock.
Flash temperatures which are extremely high localized temperatures at the microscopic level, can be much higher, exceeding the melting temperature of cobalt (1,495° C.). The presence of cobalt is believed to be the reason that PDC converts to graphite at a lower temperature than simple diamond.
When temperatures increase, graphitization of the diamond in the presence of cobalt becomes a dominant effect. Diamond wear is then due to an allotropic transformation into graphite or amorphous carbon under the influence of localized frictional heating. This transformation is accelerated in the presence of cobalt through a combination of mechanical and chemical effects. For example, the shear resistance of the cobalt drops rapidly, and the grains are not strongly held, leading to additional damage to the surface. It is also known that the real area of contact depends on the velocity with which plastic strains are propagated in the metal binder. The shearing occurs so rapidly that full plastic yielding under the normal load is not possible.
In addition, there is a significant difference between the thermal expansion coefficients of cobalt and diamond. During heating, cobalt expands at a higher rate than diamond. The amount of thermal stress in the diamond table increases, and the structure breaks down. The cobalt between the diamond crystals expands and breaks the diamond-to-diamond bonds.
The PDC cutting element therefore becomes extremely hot during drilling, however it is known that the temperature at a distance of a few microns from the contact point is about 95% of the (absolute) temperature at the point of contact. Since the temperature decreases very rapidly with increasing distance from the shearing zone (about 400 K/mm), the cutting tip behaves like a thin film of low shear strength, supported by a hard substrate. Therefore, improving the thermal stability of the cutting edge of the PDC cutting element would significantly improve drilling performance.
PDC cutters can be categorized by their abrasion resistance, impact resistance and thermal stability, and it is difficult to get all three properties maximized in one cutter variant (a cutter that is highly abrasion resistant is characterized by fine diamond particle/grain size, and a cutter that is highly impact resistant is characterized by a coarse particle/grain size).
Accordingly, there remains a need in the art for a fixed cutter bit with a cutting structure capable of enhancing bit ROP, and bit durability. As such, embodiments disclosed herein address the requirement for improved thermal stability in PDC cutting elements, and further embodiments provide PDC cutting elements with characteristics to impart high abrasive resistance and high impact strength as compared to certain conventional cutters known in the art.